s of Natural Philosophy, part
3d. where I have considered the different effects of those two kinds
of bodies upon the incident light; and, in a Dissertation upon the
Philosophy of Fire, etc. I have distinguished those two kinds of
substances in relation to their emitting, in burning, the fixed light
which had constituted a part of those inflammable and combustible
bodies.

All animals and vegetable bodies contain both those different chemical
substances united; and this phlogistic composition is an essential part
in every animal and vegetable substance. There are to be found in those
bodies particular substances, which abound more or less with one of
those species of phlogistic matter, but never is the one species of
those burning substances to be found naturally, in animal and vegetable
bodies, without being associated with the other; and it is all that the
chemical art can do to separate them in a great degree upon occasion.
Pure ardent spirit may perhaps be considered as containing the one, and
the most perfect coal the other; the chemical principle of the one
is proper carbonic matter; and of the other it is the hydrogeneous
principle, or that of inflammable air.

Thus we so far understand the composition of animal and vegetable
substances which burn or maintain our fires; we also understand the
chemical analysis of those bodies, in separating the inflammable from
the combustible substance, or the volatile from the fixed matter, the
oil from what is the proper coal. It is by distillation or evaporation,
the effect of heat, that this separatory operation is performed; and we
know no other means by which this may be done. Therefore, wherever we
find peculiar effects of that separatory operation, we have a right to
infer the proper cause.

The subject, which we are to consider in this section, is not the
composition of strata in those of mineral coal, but the transformation
of those, which had been originally inflammable bodies, into bodies
which are only combustible, an end which is to be attained by the
separation of their volatile or inflammable substances. In the last
section, I have shown of what materials the strata of mineral coal had
been originally formed; these are substances containing abundance of
inflammable oil or bitumen, as well as carbonic matter which is properly
combustible; and this is confirmed by the generality of those strata,
which, though perfectly consolidated by fusion, retain still their